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Call an API with HTTP Query Parameters

This tutorial shows you how to send a request to a Decentralized Oracle Network to call the Cryptocompare GET /data/pricemultifull API. After OCR completes off-chain computation and aggregation, it returns the asset price for ETH/USD to your smart contract. This guide also shows you how to configure HTTP query parameters to request different asset prices.

Before you begin

  1. Complete the setup steps in the Getting Started guide: The Getting Started Guide shows you how to set up your environment with the necessary tools for these tutorials. You can re-use the same consumer contract for each of these tutorials.

  2. Make sure your subscription has enough LINK to pay for your requests. Read Get Subscription details to learn how to check your subscription balance. If your subscription runs out of LINK, follow the Fund a Subscription guide.

  3. Check out the correct branch before you try this tutorial: Each tutorial is stored in a separate branch of the Chainlink Functions Starter Kit repository.

    git checkout tutorial-2

Tutorial

This tutorial is configured to get the ETH/USD price. For a detailed explanation of the code example, read the Explanation section.

  • Open Functions-request-config.js. The args value is ["ETH", "USD"], which fetches the current ETH/USD price. You can adapt args to fetch another asset price. See the CryptoCompare API docs to get the list of supported symbols. Read the request config explanation for a more detailed explanation about the request config file.
  • Open Functions-request-source.js to analyze the JavaScript source code. Read the source code explanation for a more detailed explanation.

Simulation

The Chainlink Functions Hardhat Starter Kit includes a simulator to test your Functions code on your local machine. The functions-simulate command executes your code in a local runtime environment and simulates an end-to-end fulfillment. This helps you to fix any issues before you submit your functions to a Decentralized Oracle Network.

Run the functions-simulate task to run the source code locally and make sure Functions-request-config.js and Functions-request-source.js are correctly written:

npx hardhat functions-simulate

Example:

$ npx hardhat functions-simulate
secp256k1 unavailable, reverting to browser version

__Compiling Contracts__
Nothing to compile
Duplicate definition of Transfer (Transfer(address,address,uint256,bytes), Transfer(address,address,uint256))

Executing JavaScript request source code locally...

__Console log messages from sandboxed code__
HTTP GET Request to https://min-api.cryptocompare.com/data/pricemultifull?fsyms=ETH&tsyms=USD
ETH price is: 1549.17 USD

__Output from sandboxed source code__
Output represented as a hex string: 0x0000000000000000000000000000000000000000000000000000000000025d25
Decoded as a uint256: 154917

__Simulated On-Chain Response__
Response returned to client contract represented as a hex string: 0x0000000000000000000000000000000000000000000000000000000000025d25
Decoded as a uint256: 154917

Estimated transmission cost: 0.000043407196044714 LINK (This will vary based on gas price)
Base fee: 0.0 LINK
Total estimated cost: 0.000043407196044714 LINK

Reading the output of the example above, you can see that the ETH/USD price is: 1549.17 USD. Because Solidity does not support decimals, we move the decimal point so that the value looks like an integer 154917 before returning the bytes encoded value 0x0000000000000000000000000000000000000000000000000000000000025d25 in the callback. Read the source code explanation for more information about the source code.

Request

Send a request to the Decentralized Oracle Network to fetch the asset price. Run the functions-request task with the subid (subscription ID) and contract parameters. This task passes the functions JavaScript source code, arguments, and secrets to the executeRequest function in your deployed FunctionsConsumer contract. Read the functionsConsumer section for more information about the consumer contract.

npx hardhat functions-request --subid REPLACE_SUBSCRIPTION_ID --contract REPLACE_CONSUMER_CONTRACT_ADDRESS --network REPLACE_NETWORK

Example:

$ npx hardhat functions-request --subid 6 --contract 0xa9b286E892d579dc727c79D3be9b01949796240A  --network mumbai
secp256k1 unavailable, reverting to browser version
Simulating Functions request locally...

__Console log messages from sandboxed code__
HTTP GET Request to https://min-api.cryptocompare.com/data/pricemultifull?fsyms=ETH&tsyms=USD
ETH price is: 1547.13 USD

__Output from sandboxed source code__
Output represented as a hex string: 0x0000000000000000000000000000000000000000000000000000000000025c59
Decoded as a uint256: 154713


If all 100000 callback gas is used, this request is estimated to cost 0.000052391183419864 LINK
Continue? (y) Yes / (n) No
y

Requesting new data for FunctionsConsumer contract 0xa9b286E892d579dc727c79D3be9b01949796240A on network mumbai
Waiting 2 blocks for transaction 0x0e28c3038edbfd1e3ee391fa4f93f0622a3d2dea7f8f3a06edff7669b34bd6cf to be confirmed...

Request 0xb471d27b3108cfc97345de4abca32cf3ad004cb1edfeb1a70bb55604b0ce3fa0 initiated
Waiting for fulfillment...

Transmission cost: 0.000071172820353186 LINK
Base fee: 0.0 LINK
Total cost: 0.000071172820353186 LINK

Request 0xb471d27b3108cfc97345de4abca32cf3ad004cb1edfeb1a70bb55604b0ce3fa0 fulfilled!
Response returned to client contract represented as a hex string: 0x0000000000000000000000000000000000000000000000000000000000025c37
Decoded as a uint256: 154679

The output of the example above gives you the following information:

  • The executeRequest function was successfully called in the FunctionsConsumer contract. The transaction in this example is 0x0e28c3038edbfd1e3ee391fa4f93f0622a3d2dea7f8f3a06edff7669b34bd6cf.
  • The request ID is 0xb471d27b3108cfc97345de4abca32cf3ad004cb1edfeb1a70bb55604b0ce3fa0.
  • The DON successfully fulfilled your request. The total cost was: 0.000071172820353186 LINK.
  • The consumer contract received a response in bytes with a value of 0x0000000000000000000000000000000000000000000000000000000000025c37. Decoding it off-chain to a string gives you a result: 154679.

At any time, you can run the functions-read task again with the contract parameter to read the latest received response.

npx hardhat functions-read  --contract REPLACE_CONSUMER_CONTRACT_ADDRESS --network REPLACE_NETWORK

Example:

$ npx hardhat functions-read  --contract 0xa9b286E892d579dc727c79D3be9b01949796240A --network mumbai
secp256k1 unavailable, reverting to browser version
Reading data from Functions client contract 0xa9b286E892d579dc727c79D3be9b01949796240A on network mumbai

On-chain response represented as a hex string: 0x0000000000000000000000000000000000000000000000000000000000025c37
Decoded as a uint256: 154679

Explanation

FunctionsConsumer.sol

  • To write a Chainlink Functions consumer contract, your contract must import FunctionsClient.sol. You can read the API reference: FunctionsClient.

    This contract is not available in an NPM package, so you must download and import it from within your project.

    import "./dev/functions/FunctionsClient.sol";

  • Use the Functions.sol library to get all the functions needed for building a Chainlink Functions request. You can read the API reference: Functions.

    using Functions for Functions.Request;

  • The latest request id, latest received response, and latest received error (if any) are defined as state variables. Note latestResponse and latestError are encoded as dynamically sized byte array bytes, so you will still need to decode them to read the response or error:

    bytes32 public latestRequestId;
bytes public latestResponse;
bytes public latestError;

  • We define the OCRResponse event that your smart contract will emit during the callback

    event OCRResponse(bytes32 indexed requestId, bytes result, bytes err);

  • Pass the oracle address for your network when you deploy the contract:

    constructor(address oracle) FunctionsClient(oracle)

  • At any time, you can change the oracle address by calling the updateOracleAddress function.

  • The two remaining functions are:

    • executeRequest for sending a request. It receives the JavaScript source code, encrypted secrets, list of arguments to pass to the source code, subscription id, and callback gas limit as parameters. Then:

      • It uses the Functionslibrary to initialize the request and add any passed encrypted secrets or arguments. You can read the API Reference for Initializing a request, adding secrets, and adding arguments.

        Functions.Request memory req;
req.initializeRequest(Functions.Location.Inline, Functions.CodeLanguage.JavaScript, source);
if (secrets.length > 0) {
  if (secretsLocation == Functions.Location.Inline) {
     req.addInlineSecrets(secrets);
  } else {
    req.addRemoteSecrets(secrets);
  }
}
if (args.length > 0) req.addArgs(args);

      • It sends the request to the oracle by calling the FunctionsClient sendRequest function. You can read the API reference for sending a request. Finally, it stores the request id in latestRequestId.

        bytes32 assignedReqID = sendRequest(req, subscriptionId, gasLimit);
latestRequestId = assignedReqID;

    • fulfillRequest to be invoked during the callback. This function is defined in FunctionsClient as virtual (read fulfillRequest API reference). So, your smart contract must override the function to implement the callback. The implementation of the callback is straightforward: the contract stores the latest response and error in latestResponse and latestError before emitting the OCRResponse event.

      latestResponse = response;
latestError = err;
emit OCRResponse(requestId, response, err);

Functions-request-config.js

Read the Request Configuration section for a detailed description of each setting. In this example, the settings are the following:

  • codeLocation: Location.Inline: The JavaScript code is provided within the request.
  • codeLanguage: CodeLanguage.JavaScript: The source code is developed in the JavaScript language.
  • source: fs.readFileSync("./Functions-request-source.js").toString(): The source code must be a script object. This example uses fs.readFileSync to read Functions-request-source.js and calls toString() to get the content as a string object.
  • args: ["ETH", "USD"]: These arguments are passed to the source code. This example requests the ETH/USD price.
  • expectedReturnType: ReturnType.uint256: The response received by the DON is encoded in bytes. Because the asset price is a uint256, define ReturnType.uint256 to inform users how to decode the response received by the DON.

Functions-request-source.js

To check the expected API response, you can directly paste the following URL in your browser https://min-api.cryptocompare.com/data/pricemultifull?fsyms=ETH&tsyms=USD or run the curl command in your terminal:

curl -X 'GET' \
  'https://min-api.cryptocompare.com/data/pricemultifull?fsyms=ETH&tsyms=USD' \
  -H 'accept: application/json'

The response should be similar to the following example:

{
  "RAW": {
    "ETH": {
      "USD": {
        "TYPE": "5",
        "MARKET": "CCCAGG",
        "FROMSYMBOL": "ETH",
        "TOSYMBOL": "USD",
        "FLAGS": "2049",
        "PRICE": 2867.04,
        "LASTUPDATE": 1650896942,
        "MEDIAN": 2866.2,
        "LASTVOLUME": 0.16533939,
        "LASTVOLUMETO": 474.375243849,
        "LASTTRADEID": "1072154517",
        "VOLUMEDAY": 195241.78281014622,
        "VOLUMEDAYTO": 556240560.4621655,
        "VOLUME24HOUR": 236248.94641103,
        ...
}

The price is located at RAW,ETH,USD,PRICE.

Read the JavaScript code section for a detailed explanation of how to write a compatible JavaScript source code. This JavaScript source code uses Functions.makeHttpRequest to make HTTP requests. To request the ETH/USD price, the source code calls the https://min-api.cryptocompare.com/data/pricemultifull?fsyms=ETH&tsyms=USD URL. If you read the Functions.makeHttpRequest documentation, you see that you must provide the following parameters:

  • url: https://min-api.cryptocompare.com/data/pricemultifull

  • params: The query parameters object:

    {
  fsyms: fromSymbol,
  tsyms: toSymbol
}

The fromSymbol and toSymbol values are fetched from args. See the request config section for more details.

The code is self-explanatory and has comments to help you understand all the steps. The main steps are:

  • Construct the HTTP object cryptoCompareRequest using Functions.makeHttpRequest.
  • Make the HTTP call.
  • Read the asset price from the response.
  • Return the result as a buffer using the Functions.encodeUint256 helper function. Because solidity doesn’t support decimals, multiply the result by 100 and round the result to the nearest integer. Note: Read this article if you are new to Javascript Buffers and want to understand why they are important.

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